This invention relates to mixers, and more particularly, to mixers used in direct conversion transceivers with the local oscillator (LO) self-mixing problem or the LO leakage problem being minimized.
In every kind of receiver or transmitter, mixers are always used for frequency conversion from an RF (radio frequency) to an IF (intermediate frequency) or a BB (baseband), and vice versa. A mixer used in a receiver for down frequency conversion is called a “down mixer”; and a mixer used in a transmitter for up frequency conversion is called an “up mixer”. Both up mixer and down mixer have similar structure.
Taking receivers as an example, there are numerous receiver topologies commonly used today. One of which is called a heterodyne receiver (or superheterodyne receiver), which down converts an input signal into a rather high intermediate frequency to generate an IF signal first. Then the IF signal is further filtered, amplified, and down converted again into another lower frequency (typically called a second IF or BB) for demodulation and data reconstruction. It is important that an IF filter must be used in the signal path of the heterodyne receiver, and the using of the IF filter always increases the cost and area of the whole mixer.
A direct conversion receiver (DCR) is another commonly used receiver topology, which substantially generates no IF signals. In other words, a mixer of a DCR converts the RF signals directly into baseband. FIG. 1 shows a block diagram of a conventional double-balanced mixer, which is also referred to as a “Gilbert-Cell”, used in a DCR. The double-balanced mixer 100 shown in FIG. 1 includes a current source IBIAS, six transistors Q1˜Q6, and two resistors R1 and R2. The transistor pair Q1 and Q2 converts an input signal VRF into a differential current signal IDIFF. The differential current signal IDIFF includes a first single-ended current signal IDIFF+ and a second single-ended current signal IDIFF−. With the two resistors R1 and R2 being used as loads, the four transistors Q3˜Q6 mix the differential current signal IDIFF with a LO signal VLO to generate an output signal VBB.
An advantage of the DCR is that it has a simple circuit layout. Besides, since a DCR avoids the use of IF filters, the cost of using DCR can be much lower than using heterodyne receiver with additional IF filters.
However, a DCR employing the standard mixer (such as the double-balanced mixer 100 shown in FIG. 1) suffers from a technical problem, which is called “LO self-mixing problem”. This problem is induced because the frequency of the LO signal used by the mixer is substantially the same as the frequency of the RF signal inputted into the mixer. The LO signal with voltage form might leak into the port of the mixer for receiving the RF signal and then the leaked LO signal will self-mix with the original LO signal during the mixing process. A resulted random DC offset then appears at the output port of the mixer. The random DC offset degrades the baseband signal quality and deteriorate the performance of the DCR.
Similarly, a direct conversion transmitter employing the standard mixer also suffers from a technical problem, which is called “LO leakage problem”. This problem is induced because the frequency of the LO signal used by the mixer is substantially the same as the frequency of the RF signal outputted by the mixer. The LO signal with voltage form might leak into the port of the mixer for outputting the RF signal, and then the leaked LO signal will mix with the outputted RF signal. This LO leakage problem declines signal quality of the outputted RF signal.
FIG. 2 shows a subharmonic double-balanced mixer disclosed by Rebeiz, et al. in U.S. Pat. No. 6,348,830. The subharmonic double-balanced mixer 200 shown in FIG. 2 includes a current source IBIAS, eight transistors Q1˜Q8, and two resistors R1 and R2. The four transistors Q1˜Q4 form a LO section 210; with four LO signals having four different phases and a bias current generated by the current source IBIAS being inputted, the LO section 210 generates a differential current signal IDIFF. The differential current signal IDIFF includes a first single-ended current signal IDIFF+ and a second single-ended current signal IDIFF−. The four transistors Q5˜Q8 form a RF section 220, with the two resistors R1 and R2 being used as loads, the RF section 220 mixes the differential current signal IDIFF with a RF signal VRF to generate an output signal VBB.
The subharmonic double-balanced mixer (such as that shown in FIG. 2) provides a great solution to the abovementioned LO self-mixing problem. The reason is that the frequency of the LO signals used by the subharmonic double-balanced mixer is different from that of the received RF signal. Even if the LO signals mixed with the received RF signal, since these two signals have different frequencies, DC offset problems will not be generated. Similarly, the subharmonic double-balanced mixer also provides a great solution to the abovementioned LO leakage problem. The reason is that the frequency of the LO signals used by the subharmonic double-balanced mixer is different from that of the RF signal generated by the mixer. Even if the LO signals mixed with the outputted RF signal, since these two signals have different frequencies, the leaked LO signal will not influence the signal quality of the outputted RF signal.